To meet the growing demand in portable electronic devices, energy storage devices with high specific energy, high power density, long cycle-life, low cost, and a high margin of safety may be employed. Currently, the dominant energy storage device remains the battery, particularly the lithium-ion (Li-ion) battery. Batteries store energy electrochemically, in which chemical reactions release electrical carriers that can be extracted into an electrical circuit. During discharge, the energy-containing lithium ions travel from a high-energy anode material through a separator to a low-energy cathode material. The movement of the lithium ions releases energy, which is extracted into an external circuit.
During battery charging, energy is used to move the lithium ions back to the high-energy anode compound. The charge and discharge process in batteries is a slow process, and can degrade the chemical compounds inside the battery over time. A key bottleneck in achieving enhanced performance is the limited fast-charging ability of any standard battery. Rapid charging causes accelerated degradation of the battery constituents, as well as a potential fire hazard due to a localized, over-potential build-up and increased heat generation.
For example, lithium-ion batteries currently having the highest energy density of rechargeable batteries available, typically suffer from a low power by virtue of reversible Coulombic reactions occurring at both electrodes, involving charge transfer and ion diffusion in bulk electrode materials. Since both diffusion and charge transfer are slow processes, power delivery as well as the recharge time of lithium-ion batteries is kinetically limited. As a result, batteries have a low power density, and lose their ability to retain energy throughout their lifetime due to material degradation.